1. Field of the Invention
The present invention relates to a white balance adjustment method for a light source. More particularly, the present invention relates to a white balance adjustment method and apparatus for adjusting the white balance of a light source used in a projection display system that employs a single spatial light modulator (SLM), such as digital light processor (DLP).
2. Description of the Related Art
In conventional applications that require high luminance light sources, such as projection display systems or stage lighting, gas discharge lamps are usually used. However, gas discharge lamps suffer from short lifetimes and cause environmental pollution.
A more environmentally-friendly choice for this type of light source is solid state light (SSL) sources based on laser diodes (LDs) or light emitting diodes (LEDs). Currently, there are two types of SSL sources that can be used in projection displays. The first is the direct light emitting devices. Another is a hybrid approach, which uses a blue or ultraviolet (UV) light emitting device to excite a wavelength conversion material such as phosphors. Both of these two types of light sources have long lifetime, better energy efficiency and less environmental pollution. As a result, SSL sources are good candidates for the next generation high luminance light sources that can replace conventional gas discharge lamps.
In replacing gas discharge lamps by SSL sources, it may be desirable to keep many components of the current projection systems unchanged, so that many existing equipment, products and designs can be adapted for use with SSL sources, which is both environment friendly and helps the projection display business adopt this new light source technology more quickly. However, this may lead to a problem of color balance. As seen in FIG. 1, a hybrid SSL source has a different spectrum than an ultra high performance (UHP) lamp. If a SSL source replaces a UHP lamp in a projector, the output red, green and blue colors may be only slightly different than those provided by the UHP lamp, but the luminous flux proportion of the red, green and blue colors may be very different from that of the traditional projector using UHP lamp, which will result in very different white color balance.
In more detail, FIG. 2 is a schematic view of a conventional single DLP projection system. The UHP lamp 201 generates a white light which is collected by a reflector 202 and condensed by a lens 203. A rotating color wheel 204 has different filter segments that allow red (R), green (G) and blue (B) light to pass through. When the wheel rotates, different filter segments are sequentially located in the path of the white light from the lens 203 to output a predetermined repeating sequence of color lights. The different colored light sequentially arrives at a spatial light modulator 210 through a series of optics like integration rod 205, lenses 206, 207 and 208, and a TIR (total internal reflection) prism 209. The spatially modulated colored light is directed to a projection lens 211 and forms an image on a screen.
If the UHP lamp is replaced by a SSL source 301, as seen in FIG. 3, the white balance and luminance may change since the luminous flux ratio of the R, G and B light and the radiant flux of the SSL source may be different from those of the UHP lamp. White balance and luminance adjustment are desired for such a system.
Moreover, in current projection system using SSL sources, white balance and luminance may vary as a result of aging, temperature changes, and other factors. In addition, customers may desire a projection system that has tunable white balance and luminance according to their preferences or needs. Such systems also need adjustment of white balance and luminance.